phagocytes like macrophages have general receptors to recognize some pathogens, and engulf them whole. T cells recognize the antigens, which is only part of the pathogen, but the antigen has to be presented on the host cells. You are mixing up the order of antibodies and T cells. Antibodies come from B cells, but cant be activated until the T cells turn them on.

A Phagocyte is any cell that engages in phagocytosis. So, I'd guess you mean the difference between Leukocytes and Lymphocytes.

Leukocytes are(basically) Macrophages, Neutrophils, and Monocytes.

Lymphocytes are(basically) T-Cells and B-Cells.

Both are parts of the immune system and neutralize antigens.

An antigen is anything that causes an immuneresponse, which isn't necessarily a pathogen. Infact, many antigens cause no harm at all, which causes an allergic reaction.

So, a very basic summary of the immuneresponse is:

A pathogen bypasses the body's physical and chemical barriers and the inflammation response. The pathogen is engulfed by a macrophage. The macrophage forces the surface proteins of the pathogen(antigens) to it's surface. Helper T-Cells get the information on how to neutalize the pathogen from the macrophage(from the antigens of the pathogen). These Helper T-Cells begin to multply and have two main roles. The first is to activate B-Cells and "tell" them how to neutralize the pathogen(antibodies). The B-Cells(also called Plasma cells) begin to multiply and produce the antibodies to neutralize this specific pathogen. The second role is to activate the Killer T-Cells. These Killer T-Cells can either destroy the pathogen, or, more commonly, destroy infected tissue. Since this consumes a lot of energy, when the process is complete, Supressor T-Cells stop the process completely, but also "tell" some B-Cells to "remember" how to destroy that specific pathogen, and become Memory B-Cells.

Just a small correction:
A phagocyte is a generic term for any cell that can carry out phagocytosis. In the human immune system, there are 4 main types of phagocytes: macrophages(monocites in the blood), neutrophils, dendritic cells and eosinophils(Yes, eosinophils do have phagocytic abilities, although poor).
The human body contains many other cells with the capacity of phagocytosis that are not part of the immune system: septal cells in lungs, Kupfer cells in the liver, oligodendrocytes in the brain, osteoclasts in bones etc
Regards,
Andrew

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

Yo, you seem to forget that B-cells are also phagocytes. Crosslinking of membrane bound antibodies will cause an internalization of the antibody-antigen complex, processing of the antigen, and presentation of that particular processed antigen on class II MHC. This will allow it to interact and activate T-Helper cells via the T-cell receptor specific to that processed antigen in the context of class II MHC.

the B cell does take in the antigen and then present it to a helper T cell by a class II MHC molecule, however the B cell is not a phagocyte. The process by which the B cell takes in the antigen is not phagocytosis, but receptor mediated endocytosis. The main diference in principle is that receptor mediated endocytosis only takes in specific molecules, while phagocytosis takes in all kinds of things. That is why a B cell can only present one antigen, the one it has receptors for.

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

I just joined this forum. I tried to use one of the two facts, biology noivce, as my username, but it was already taken. Then I used the other, here by chance.

I liked your clarification for phagocytosis and endocytosis - answered one of my long time questions. I have some further questions if you don't mind:

1. Does B cell process the endocyted molecules in similar way as phagocytes do, so MHC II present pieces of the molecules, or MHC II in B cell present whole molecules as they are endocyted?

2. If an antigenic molecule that a B cell receptor can recognize is bound to the surface of a, say, bateria body, does B cell endocyte the entire bacteria body?

3. If the answer to question 2 is yes, is B cell capable to lyse and digest (process) the bacteria, and presend small pieces by MHCII on surface like the phagocytes do?

4. If the answers to 1-3 are all yes, then the end results, or functions, of phagocytosis by phagocytes and endocytosis by B cells don't seem to be much different. Am I missing something?

5. (unrelated to the above) I was given the impression that a T helper cell gets activated when its receptors recognize antigens presented by MHC II on some phagocytes. What's not clear to me is that when the activated T helper cell move on to activate B cells, does it require the B cells to have the same antigen presented on surface? If not, how the T helper cell "tell" the B cells what antibody to make? If yes, the chance for all events to happen seems to be very very low to me.

6. Finally, if you don't mind, why B cells developed T cell-dependent and T cell-independed pathways to produce antibodies? What are the advantages and disadvantages of each?

well...phagocytes are mainly part of the immune systems defense system, like when there is an inflammatory response. While lymphocytes are mainly for when the immune system attacks a foreign organism. However, lymphocytes can become phagocytes thats why i didnt say that they were part of a certain sector of the immune system.

herebychance wrote:I just joined this forum. I tried to use one of the two facts, biology noivce, as my username, but it was already taken. Then I used the other, here by chance.

I liked your clarification for phagocytosis and endocytosis - answered one of my long time questions. I have some further questions if you don't mind:

1. Does B cell process the endocyted molecules in similar way as phagocytes do, so MHC II present pieces of the molecules, or MHC II in B cell present whole molecules as they are endocyted?

2. If an antigenic molecule that a B cell receptor can recognize is bound to the surface of a, say, bateria body, does B cell endocyte the entire bacteria body?

3. If the answer to question 2 is yes, is B cell capable to lyse and digest (process) the bacteria, and presend small pieces by MHCII on surface like the phagocytes do?

4. If the answers to 1-3 are all yes, then the end results, or functions, of phagocytosis by phagocytes and endocytosis by B cells don't seem to be much different. Am I missing something?

5. (unrelated to the above) I was given the impression that a T helper cell gets activated when its receptors recognize antigens presented by MHC II on some phagocytes. What's not clear to me is that when the activated T helper cell move on to activate B cells, does it require the B cells to have the same antigen presented on surface? If not, how the T helper cell "tell" the B cells what antibody to make? If yes, the chance for all events to happen seems to be very very low to me.

6. Finally, if you don't mind, why B cells developed T cell-dependent and T cell-independed pathways to produce antibodies? What are the advantages and disadvantages of each?

Hope your answer won't be "read an immunology textbook"

Regards,

A_biology_novice_got_here_by_chance

As was mentioned earlier, B cells engulf cells by receptor-mediated endocytosis. Phagocytes engulf the entire pathogen inside food vacuoles and kill the pathogens and release antigens, or present them to B cells through class II MHC molecules. B cells bind *free or presented* antigens on their B-cell receptors and internalize them, and then present them in their own class II MHC molecules. This magnifies the immuneresponse. Through their MHC II molecules, they present the antigen to either cytotoxic T cells, which can bind to and lyse infected cells, or helper T cells, which can secrete cytokines to activate B cells and cytotoxic T cells to further magnify the immuneresponse. I am not entirely sure, but I think other B cells have to have the same antigen presented, and it binds to them via a CD4 molecule (remember a helper T cell has several CD4 molecules). Activation of B cells via cytokines from helper T cells cause the proliferation of B cells into plasma cells and memory B cells. Plasma cells produce antibodies. Memory B cells help in the secondary immuneresponse: they can produce plasma cells whenever the body is exposed to the same antigen again. So the T-cell dependent pathway ensures that the signal is magnified by contact with helper T cells before the B cell proliferates into plasma cells, and the T-cell independent pathway provides a quicker way to respond to the antigen if the body is infected again.

Aptitude, that was a brilliant response. I was in microbiology class yesterday biting my tongue while the professor kept referring to B cells as phagocytes. Biology is all about using the proper terminology!